1. Field of the Invention
This invention relates to a method and related equipment for preforming the edges of sections of strip of plate prior to forming and welding them into large diameter pipe. It is particularly useful for pipe made from thick gauge plates according to the so-called U-O process.
The U-O process for making large diameter pipe was developed primarily to produce pipe used in the oil and gas industry to transport these products over long distances. It is not unusual for such pipe line to be several hundreds of miles long. The ever increasing demand for these energy related products makes it desirable to transport them through pipe lines at much higher pressures and over a broader range of geographical temperature conditions. This requires pipe wall thicknesses in excess of one inch and in grades of steel that are very hard to form.
By the U-O pipe making system individual sections of flat steel plate or strip that are cut to length are formed into U-shaped sections by a hydraulic press or the like which is known as a U-Press. The U-shaped section of strip or plate is then subjected to a so-called O-Press operation wherein another press, also usually of the hydraulic type, forms the section into an O-shape approximating the final configuration of the pipe. Such section or pipe unit is thereafter welded or otherwise sealed by conventional methods at its abutting edges. Typical examples of U-Presses and O-Presses disclosing the system referred to above are U.S. Pat. Nos. 2,591,085 and 2,588,325.
One of the most difficult and costly problems in forming pipe sections or units by the U-O system, and particularly from heavy gauge metal, is that of forming of the edges along the length of the section that each edge is formed to the proper radius to the bitter edge to conform to the desired over all diameter of the pipe section. The normal practice is to preform the edges of the section to be formed into pipe prior to subjecting the section to the U-Press or O-Press operation. With the employment of heavier gauge plate sections and hard to form grades of steel, the preforming of the edges by systems now in use have become a difficult and expensive operation requiring high capital expense and cumbersome crimping and forming equipment.
The usual practice in forming large diameter pipe from a section of plate is to cut the plate section to the required width for the ultimate pipe diameter. Thereafter, edge planers, planishing rolls or other means may be used to bevel the edges of "noses" of the long edges of the plate section while it is still flat so that such edges are of a suitable shape for welding following the U-O pipe-making operation. The plate section edges are then formed to the radius of the finished pipe. This is done by passing the plate edges between edge crimping rolls with the proper radius as closely as possible by heavy presses (generally hydraulic) fitted with properly contoured dies. On heavy plate, these methods cannot bend the plate to the extreme edge since the principle involved in plate bending requires that some of the plate edge be used as a lever arm to attain the requisite bending moment to bend the plate. Under such conditions the width of the plate edge that acts as the lever arm to bend the plate adjacent to the bend remains as an unbent flat edge. The width of this flat edge is dependent upon the strength of the crimping rolls or press which exerts the force required to bend the plate. In order to reduce the length of the remaining lever arm, that is the area that functions as a lever arm, the bending forces exerted must be materially increased to obtain and maintain the requisite bending moment. This manner of attaining reduction in the length of the lever arm is limited by the outermost edge of the crushing resistance of the plate to the increased forces. Presses built to exert such forces are very expensive and they still fall short of providing the ideal edge bend conditions.
Conventional methods of bending or crimping the edges of plate or strip are consequently not effective adjacent the bitter edges of a plate section. There remains a defective, flat area at each edge approximating the thickness of the plate. This is the area that cannot be bent. In fact the required forces to do so would cause crushing of the plate. Consequently, thicker plates result in wider and longer unformed areas or regions which cause a defective pipe contour when the edges of the plate section are brought together for welding following the U-O operation. The flat area adjacent to the edge of the welded seam of the pipe results in a peaked contour which is undesirable and can lead to defects such as "toe cracks" on the inside weld of the pipe when operated under pressure.
2. Description of Prior Art
In prior art U-O pipe making operations, it has always been the practice to first trim plate or sections to a width corresponding to the diameter of the pipe which is ultimately to be formed. In other words, the sections of metal to be formed into pipe are first cut and planed to a width that is the same as the periphery of the pipe to be formed.
Typical examples of prior art wherein edge bending has been practiced by roll-forming has been confined to strip or other relatively thin gauge metal under 1" thick. Typical of this practice is U.S. Pat. No. 3,635,064 to Chang where the edge or the pipe sections are edge formed after the strip has been partially formed. The strip for the tube or pipe is cut to the required width to suit the diameter of the tubing or pipe prior to introduction to the mill and any forming operation. No suggestion is made for slitting or cutting the edges of the strip or plate after forming commences.
U.S. Pat. No. 3,794,409 discloses edge bending strip prior to formation of helical pipe. The object is to compensate in advance for the ultimate edge distortion which occurs in the fabrication of helical pipe. Helical pipe is not made by the U-O process and is not subject to U-ing and O-ing operations and plate sections are formed into different shapes.
There are other examples in the prior art showing other methods of bending the edges of skelp or strip. However, it is apparent that in such instances, prior to any forming, the section or work is sheared to a width intended to provide pipe or tubing of the diameter ultimately desired. U.S. Pat. No. 3,430,475 to Lindmark is to tubing that is first roll formed into a shape approaching the final product prior to any edge bending operation. Another patent, U.S. Pat. No. 4,122,696, discloses the practice of holding the edges in contact with the finishing rolls to avoid warping as the material passes through U-ing and O-ing roll-forming operations. The edge bending is not a separate preforming operation.